1. Field of the Invention
The invention relates to cardiopulmonary bypass procedures, and more particularly, to controlling patient temperature in conjunction with said procedures.
2. Description of Related Art
Cardiopulmonary bypass surgery (CPB) is one of the most common surgical procedures performed in the United States. During CPB, the heart is stopped and the blood which normally returns to the right side of the heart passes through a pump and oxygenating system and is returned to the aorta, thereby bypassing the heart and lungs.
Primary goals of cardiopulmonary bypass for heart surgery are to provide life-support functions, a motionless, decompressed heart, and a dry, bloodless field of view for the surgeon. In a basic heart-lung life-support system oxygen-poor blood is diverted from the venous circulation of the patient and is transported to the heart-lung machine where reoxygenation occurs, carbon dioxide is discarded and heat regulation (warming or cooling) is accomplished. This processed blood is then returned (perfused) into the patient""s arterial circulation for distribution throughout the entire body to nourish and maintain viability of the vital organs.
Although a common procedure (in excess of 400,000 open heart procedures per annum are conducted in North America) and although tremendous strides have been made so that open heart surgery is safer for patients, the procedure is not without its dangers. Further, while the vast majority of patients have marked improvement in their cardiac functional status following their procedure, of concern is the potential for damage to other organ systems which can result from the CPB procedure.
Particularly, time on bypass is positively and independently correlated to adverse outcome of CPB, and reducing pump time is a clinically meaningful measure of device performance. Neurocognitive deficits are associated with CPB and attributed to emboli in the arterial circulation inevitably associated with arterial cannulation, surgical procedures and large, complex extracorporeal devices.
Neurologic and other embolic related sequelae are typical for surgery where CPB is used. The reason for these problems is that emboli from various sources are launched into the arterial circulation as a result of extracorporeal circulation and procedures inside the heart. These emboli are in the arterial circulation and pass into the major organ systems throughout the body, without benefit of capture by the lungs. Emboli larger that blood cells (8-15 microns) lodge in the arterioles and capillaries and cause ischemic areas corresponding to the areas perfused by the occluded blood vessel.
Conventionally, various means are employed to either prevent the formation and release of emboli into the arterial blood circulation or filter or trap blood-borne emboli prior to infusion into the patient""s arterial circulation. Examples of filters and traps are screen or depth type filters in the extracorporeal blood circuit. These filters or traps may be in reservoirs, integral to blood gas exchange devices (oxygenators), cardiotomy reservoirs, and arterial line filters. Antithrombotic coatings may be applied to extracorporeal devices and cannulae to prevent thromboemboli. Carbon dioxide flushes may be used to displace air (carbon dioxide is much more soluble in blood than air) from extracorporeal circuits and reduce the potential for air bubbles. Not withstanding the above measures, emboli and the associated neurologic sequelae are a feature of CPB.
Filtration methods for removing emboli from blood are limited by the cellular nature of blood and the blood""s propensity to form thrombi when exposed to artificial surfaces and/or shear forces. As a filter""s pore size approaches that of the blood cells (8-15 microns) the pressures needed to achieve sufficient flow are increased or the area of the filter must be increased to impractically large size. Additionally, as the shear forces adjacent to artificial surfaces increase, platelet aggregates and/or fibrin thrombi formation ensue on the downstream side of the filter and, paradoxically, create blood emboli.
Actions and manipulations of the patient and equipment for CPB cause emboli in the arterial blood flow. It has not been possible to practically eliminate all the emboli so caused. For example, insertions of the venous and arterial cannulae cause small pieces of cut or torn tissue to enter the blood. Cardiotomy suction blood (typically filtered and returned to the CPB circuit) has air, fat and tissue emboli that can only be partially filtered (for reasons previously mentioned) out of the blood before going into the arterial circulation. As part of CPB, large clamps are applied and released at various times to stop/start blood flow in major blood vessels and this action causes damage to the blood vessel lumen and creates stagnant, clot prone areas near the clamp. Subsequent movement and eventual release of clamps has been shown to launch measurable emboli into the arterial (including cerebral) circulation.
Cooling the patient is routinely employed in CPB. This is accomplished by heat exchangers in the extracorporeal circuit. The benefits of cooling to protect the patient from ischemic insult are well recognized. However, conventionally the patient is at normal temperature at the time that CPB is initiated and no cooling protection is afforded until after the initial embolic insult. 10-20 minutes may be required after the start of CPB to reach the desired hypothermic temperature. Significant emboli (tissue, particulate, air and thrombus) are released at the precise time that CPB is initiated and the patient has not yet cooled below the normothermic range (36.0-37.5xc2x0 C.).
Cooling also provides the benefit of increasing the margin of safety in case of equipment failure, whereby patient metabolism is reduced by the cooled blood being reintroduced into the body, in turn reducing the body""s need for oxygen and the tolerance for its deprivation in the event of such failure. Specifically, an 8-10% decrease in oxygen consumption is correlated with each degree (Celcius) drop in body temperature. Thus it is not uncommon to lower patient core body temperature to about 32xc2x0 C., or even lower, during CPB.
The present invention relates to methods and systems for controlling patient temperature during cardiopulmonary bypass surgery. In one embodiment, a patient undergoing cardiopulmonary bypass surgery is precooled using an indwelling catheter inserted into the central venous system of the patient. The indwelling catheter operates to cool the patient, lowering core body temperature before cardiopulmonary bypass is initiated. A pre-determined patient target temperature can be set so that cooling is terminated or patient target temperature is automatically maintained when the target temperature is reached.
In a second embodiment, the indwelling catheter can be used to control the rate of patient rewarming once the cardiopulmonary bypass surgery is near or at completion. Controlled rewarming can be encompassed by applying heat to the patient""s blood flow using the indwelling catheter. Additionally, the caregiver can control the rate at which the patient is rewarmed by selecting the pump speed and bath temperature of the temperature control module.
It is the object of the present invention to provide a systemic heat exchange method and system that are effective, are easy to use and require minimal added work for medical personnel. Additional objects and advantages of the invention will be set forth in part in the description which follows, and may be obvious from the description or learned by practice of the invention. The objects and advantages of the invention also may be realized and attained by means of the method acts, instrumentalities and combinations particularly pointed out in the appended claims.